1. Field of the Invention
The present invention relates to the field of proximity sensors; and more particularly to field disturbance sensor technology.
2. Description of Related Art
Field disturbance sensors provide a class of motion detectors which have a wide range of applications, such as automotive burglar alarms, house intrusion safety and robotic sensors, industrial counting and process control, automatic door openers, and automotive obstacle detection.
These sensors operate basically by transmitting an electromagnetic signal, and detecting reflected energy in a sensor field. The reflected energy in the sensor field will reach a steady state, when there is no motion within the field. If an object which absorbs or reflects the electromagnetic energy enters the field, then changes in the reflected energy are detected. Field disturbance sensors operating at microwave frequencies are essentially identical to continuous wave (CW) Doppler microwave sensors.
Existing designs suffer several significant problems which limit their application. In particular, they are subject to false alarms caused by interference from other transmitters close to the field of the sensor, or caused by oversensitivity of this type of sensor at close range.
The field disturbance sensors of the prior art rely on relatively high power microwave energy in most applications. These high power microwave applications are limited by communication regulations to a relatively narrow band, which is quite crowded. For instance, the existing field disturbance sensors often operate in the same frequency band as microwave ovens and other high power microwave devices. This subjects the sensors to false readings caused by external transmitters in the range of the receiver on the sensor. Because the allocated frequency range for such uses is relatively narrow, the number of channels which can be used for these purposes is limited. Thus, the number of sensors which may be used in a given field is limited.
Field disturbance sensors of the prior art are also hypersensitive at close range. Thus, a sensor which is tuned to detect motion of a person at about 10 feet, will emit a false alarm when an insect lands on the surface of the antenna. This problem arises because the sensitivity of the device falls off as function of 1/R.sup.2, where R is the distance from the transmitter to the reflecting object. Also, this hypersensitivity subjects the sensors to false alarms from vibrations or other mechanical disturbances to the sensor.
Accordingly, it is desirable to provide a field disturbance sensor which overcomes the false alarm problems of prior art designs. Also, it is desirable to provide a system which allows multiple sensors to be placed in a single field without interference.